The present invention relates to a battery capacity calculating method, a battery capacity calculating apparatus and a battery capacity calculating program, all being for calculating residual capacities and/or residual power of a secondary battery.
For example, like a notebook type personal computer and a portable telephone, an electronic apparatus having a secondary battery (hereinafter simply referred to as a battery), such as a lithium ion secondary battery, as its power supply has been spread recently.
As a method for calculating a residual capacity of a battery, various methods have been proposed and examined. They are, for example, a method using a relation between terminal voltages and residual capacities of a battery, and a method of subtracting a discharged capacity obtained by integrating discharging currents from a fully charged capacity in the case where the discharged capacity at the time of the fully charged capacity is defined as “0”.
Here, the relations between the terminal voltages and the residual capacities can be expressed by curves Cnew and Cold along which the terminal voltages decrease to a predetermined discharge stopping voltage Vcut as the residual capacities decrease, as shown in FIG. 16, for example. In particular, in a battery, the terminal voltage thereof has a tendency of falling rapidly when the residual capacity becomes in the neighborhood of “0”, namely in the last stage of discharging. Accordingly, a predetermined discharge warning voltage Vwar is set in this region. Incidentally, in the drawing, a curve Cnew expresses the terminal voltages of an initial battery which has not deteriorated, and a curve Cold expresses the terminal voltages of a deteriorated battery which has deteriorated by use. That is, in the battery, it is known that, when the battery was used for a long period of time and has been deteriorated, the relation between the terminal voltages and the residual capacities of the battery significantly changes, even if the discharging conditions are the same. Therefore, a problem of decreasing the estimation accuracy of the residual capacity is pointed out in a battery capacity calculating method using the above-mentioned relation between the terminal voltages and the residual capacities of a battery.
Accordingly, it is necessary to take into consideration a measure to the changes of the voltage characteristic owing to such a deterioration of a battery on calculating the residual capacity of the battery. As for the above-mentioned battery capacity calculating method of subtracting the discharged capacity obtained by integrating the discharging currents from the fully charged capacity, various methods have been examined as a method of coping with the changes of the voltage characteristic owing to the deterioration of a battery (see, for example, Japanese Patent Application Laid-Open Publications No. 2002-236154 and No. 2002-162452). They are, for example, a method of counting the number of the cycles of charging and discharging and correcting the fully charged capacity of a battery according to the counted number of the cycles, a method of correcting the fully charged capacity of a battery using a charged capacity until the battery becomes in its fully charged state, and the like.
Japanese Patent Application Laid-Open Publication No. 2002-236154 discloses “a residual capacity correcting method of a battery” as a method of counting the number of the cycles of charging and discharging to correct the fully charged capacity of the battery according to the counted number of the cycles. The Japanese Patent Application Laid-Open Publication No. 2002-236154 describes that not only the number of the cycles but also the display error of the residual capacity can be lessened, even if a battery is used after being saved for a long period of time, by correcting the fully charged capacity of the battery according to the saving state thereof.
Moreover, the Japanese Patent Application Laid-Open Publication No. 2002-162452 discloses “a battery residual quantity detecting circuit” as a method of correcting the fully charged capacity of a battery using a charged capacity until the battery becomes the fully charged state. The Japanese Patent Application Laid-Open Publication No. 2002-162452 describes that the residual capacity of a battery can be detected accurately by correcting fully charged capacity using the charged capacity to the fully charged state at the time of discharging to predetermined discharge warning voltage even if the voltage characteristic of the battery has changed.
However, the technique described in the Japanese Patent Application Laid-Open Publication No. 2002-236154 includes a problem in which a value different from the actual fully charged capacity is calculated owing to the variety of the actual use environments and the actual deterioration forms if the fully charged capacity is corrected using a correction coefficient measured in ideal environment.
Moreover, there are the following problems in the technique described in the Japanese Patent Application Laid-Open Publication No. 2002-162452.
Since an electronic apparatus connected to a battery usually shifts to the termination processing thereof with the battery having a residual capacity necessary for the termination processing, the discharge warning voltage is set to be higher rather than the original discharge stopping voltage of the battery. Here, although the discharge warning voltage is determined using the measured data of the initial battery, which has not deteriorated, the inclination of the curve in the last stage of discharging becomes gentle as the battery becomes deteriorated as shown in FIG. 16. In other words, the inclination of the discharge curve of a battery in the last stage of discharging, which shows the relation of the discharged capacities and the terminal voltages of the battery, becomes gentle as the battery becomes deteriorated. Consequently, even if the voltage of the deteriorated battery is the discharge warning voltage or less which has been determined based on an initial battery which has not deteriorated yet, a sufficient capacity remains in the deteriorated battery.
However, because the technique described in the Japanese Patent Application Laid-Open Publication No. 2002-162452 cannot lower the discharge warning voltage according to the deteriorated state of a battery, the fully charged capacity is underestimated. Then, there is a problem in which, even if a battery has not actually deteriorated so much, the residual capacity thereof is calculated too little, and the real use time of an electronic apparatus becomes shorter, as a result.
On the other hand, as a method of calculating the residual capacity of a battery from a viewpoint different from the method using the relation between the terminal voltages and the residual capacities and the method of subtracting the discharged capacity obtained by integrating the discharging currents from the fully charged capacity as discloses in the Japanese Patent Application Laid-Open Publications No. 2002-236154 and No. 2002-162452, there is a technique described in Japanese Patent Application Laid-Open Publication No. 2001-231179.
The Japanese Patent Application Laid-Open Publication No. 2001-231179 discloses “a battery capacity detecting method, and an apparatus and a battery pack” as a method of calculating a residual capacity using the internal impedance and the equilibrium voltage of a battery. Incidentally, the equilibrium voltage indicates a terminal voltage of a battery when the internal states of the electrode and the electrolytic solution of the battery become stable after the battery has been left in the state in which the terminals of the battery were opened for a long period of time. The Japanese Patent Application Laid-Open Publication No. 2001-231179 discloses a technique of measuring an equilibrium voltage curve which shows the relation of the discharged capacities and the equilibrium voltages of a deteriorated battery, and the internal impedance thereof to calculate the residual capacity of the battery based on the voltage drop by the internal impedance. The Japanese Patent Application Laid-Open Publication No. 2001-231179 also describes that the applicant of the application tried to calculate not only the residual capacity but also accurate residual power.
However, the technique described in the Japanese Patent Application Laid-Open Publication No. 2001-231179 includes the following problem.
The voltage drops ΔV owing to the internal impedance of a battery is expressed by the differences of the equilibrium voltage curve Cequ and the discharge curve Cdch as shown in FIG. 17. The internal impedance changes in dependence on discharged capacities, and significantly increases in the last stage of discharging. Therefore, in order to calculate residual capacities using the internal impedance, it becomes the most important matter to take into consideration the increase of the internal impedance in the last stage of discharging.
Now, it is known that the internal impedance of a battery also changes as the battery becomes deteriorated. The technique described in the Japanese Patent Application Laid-Open Publication No. 2001-231179, tries to calculate residual power by measuring the internal impedance suitably. In particular, in order to measure the increase of the internal impedance in the last stage of discharging, it is necessary to perform discharging completely till the last stage of discharging.
That is, because the technique described in the Japanese Patent Application Laid-Open Publication No. 2001-231179 cannot grasp the internal impedance in the last stage of discharging during discharging, the technique has a problem in which a residual capacity cannot be accurately estimated in the case where the internal impedance measured in advance has significantly changed in comparison with the present internal impedance such as a case where a battery begins to discharge from the state saved for a long period of time and a case where a battery has deteriorated by the repetition of charging and discharging at a shallow depth of a discharge.
As mentioned above, these various conventional battery capacity calculating method cannot previously estimate the discharge curve of a deteriorated battery especially in the last stage of discharging. Consequently, there is a problem in which, in the case where a residual capacity is calculated using the conventional battery capacity calculating method, the capacity of the battery cannot be used up completely and then the real use time of an electronic apparatus became shorter.
That is, the conventional battery capacity calculating methods severally have a contradictory problem in which the methods cannot calculate an accurate residual capacity until a battery has discharged almost completely, and it is needed to grasp the accurate residual capacity beforehand in order to perform almost complete discharging on the other hand. This fact means that the conventional battery capacity calculating methods are actually unrealizable.
Moreover, by the conventional battery capacity calculating methods, the situation of being unable to detect the true deteriorated state of a battery and detecting the deterioration of the battery excessively because the capacity of the battery cannot be used completely is also caused.